In modern healthcare, clinical measurements and medical data collection are abundantly performed on a patient in order to obtain information for medical research, diagnostic testing, early detection of critical illnesses and warning thereof, analysis of disease as well as monitoring the effect of medical treatment. In general terms, the purpose of clinical measurements is to collect information on the various body functions and or possible physiological conditions.
Information on body functions may for example be obtained by measuring the so called “biopotentials”. A biopotential is an electric potential difference measured between two regions on, in, or near a patient's body.
An example of such a biopotential is the electrocardiogram (ECG). An ECG is a measurement in which the electrical conductivity of the nerve bundles located in the heart is determined. Due to the activity of the heart muscle, this conductivity varies in time. Based on obtained electrical signals representing the ECG, one can establish whether the temporal activity of the heart is normal or anomalous.
The ECG is obtained by applying a set of electrical sensors near, to, or inside of the patient's body. The electrical signal measured by the sensors is sent to a processing device. The sensor device and processing device combined form an acquisition system for measuring an ECG biopotential.
Other examples of biopotentials are the electromyogram (EMG), the electroencephalogram (EEG), the electrooculogram (EOG) and the vector cardiogram (VCG).
Commonly, biopotentials are measured by affixing several sensors with measuring electrodes to body regions of a patient. At least one sensor functions as a reference sensor. This reference sensor provides a reference potential measurement and is placed on a measurement portion of the patient body that is expected to be minimally electrically active. Subsequently, the potential difference between the reference sensor and any other sensor is determined. Alternatively, differences between the measured biopotential signals by pairs of sensors may be determined. Such a differential measurement is referred to as a “lead”.
Measured potential differences are often very small, typically having a signal amplitude in the order of micro- to milliVolts. Therefore, distinguishing biopotential signals from the background electrical noise requires considerable signal processing.
Typically, the various sensors in the sensor device are connected to the processing device by means of electrically conducting cables. The sensor device transmits the measured biopotentials through the cables to the processing device, which processes the received biopotentials and possibly stores them in a buffer.
The biopotential signals measured with known systems are sensitive to various types of electrically induced noise. Capacitive effects and the tendency of the cables to act as receiving antennas for ambient EM fields may severely reduce the signal to noise ratio of the biopotentials. Capacitive coupling is caused by relative motion of the cables and the sensors that are attached to the patient's body. Random movement of a patient, e.g. during sleep or medical transport, may cause such effects. The interaction of RF-fields (e.g. from mobile phones) with the various components of the processing device constitutes an additional source of noise. Furthermore, ambient field sources operating at the mains supply frequencies, local temporary distortion of the common ground potential, or ground loop circular currents occurring between the patient's body and the processing device will all add further noise to the biopotential measurements.